To reduce the percentage of nitrogen oxides in the exhaust gas discharged by a diesel internal combustion engine, it is known that a reactant supporting the selective catalytic reduction can be introduced into the exhaust gas stream. It is necessary in this connection for carrying out the catalytic reaction effectively to bring about effective mixing of the reactant with the exhaust gas flowing in an exhaust system. In general, a mixer, which shall bring about a swirling with a plurality of flow deflection surfaces in the exhaust gas stream and hence improved mixing of the exhaust gas with the reactant injected into the exhaust gas, in general, in the form of a spray, is arranged for this, in general, downstream of the location at which the reactant is introduced.